1. Field of the Invention
The present invention is directed generally to a device intended to replace or partially replace a vertebral body or intervertebral disc in the cervical, thoracic or lumbar spine, as well as methods for its use and deployment.
2. Description of the Related Art
All publications cited herein are incorporated by reference in their entirety to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Various devices are known in the art for the replacement or partial replacement of vertebral bodies or intervertebral discs in the cervical, thoracic, and lumbar spine. These are often used to replace, restore, correct, augment, strengthen, support or maintain height of an unstable, fractured, collapsed, damaged, degenerated or degraded vertebral body segment or intervertebral disc space or portion thereof, which has compromised integrity due to cancer or tumor or other lesion including hematalogic, metabolic or other systemic processes affecting spinal structures; is unstable because of trauma or any other process compromising integrity of ligaments, soft tissues, or osseous structures; or is characterized by a degenerative or deforming process including but not limited to congenital deformities, degenerative disease, kyphosis, spinal deformity/scoliosis, or spondylolisthesis. The device described herein is also relevant and applicable as it pertains to any spinal segment that has a surgical indication based on patient presentation and surgeon's judgment with no limitations otherwise. Cervical neck pain or axial back pain with or without radicular symptoms are frequently symptoms, and neurologic deficit involving loss of strength or sensation is frequently a sign, of such conditions. Further devices and methods are known in the art for performing the surgical procedures required to effectuate the removal of disc tissue or osseous structures and achieve the insertion and placement of some type of spinal segment replacement device.
The number of thoracic and lumbar fusions being performed has increased dramatically during the past ten years, far in excess of what would be expected proportionately considering population growth and aging demographics alone. Spine surgeons are now seeing many patients return post-fusion with symptoms limiting their professional, personal, and social functionality that stem from a fusion performed in the past without appropriate consideration for the biomechanical principle of sagittal balance and restoration of lumbar lordosis. The vast majority of thoracic and lumbar fusions are currently performed through a posterior approach, through a posterolateral fusion that does not involve interbody grafting or via a posterior lumbar interbody fusion (“PLIF”) cage (i.e., the disc space is accessed bilaterally and extensive nerve root retraction is required) or through a transforaminal lumbar interbody fusion (“TLIF”) with a TLIF cage (i.e., the spine is approached posteriorly similar to the PLIF, but unilaterally more from the side of the spinal canal through the foramen, potentially reducing nerve root retraction required to access the disc). The traditional posterior approach is well-tolerated by patients but can restrict the amount of disc material that can be removed safely and more importantly restricts the size and type of currently available cage that can be placed through this small opening, an orifice limited by the degree of traction that can be placed on critical nerve roots or otherwise limited by the window of access to the disc space. Current grafts available for insertion via a posterior approach are hampered by subphysiologic degrees of lordosis and endplate coverage, compared with anatomically appropriate lordotic grafts with larger endplate coverage. Given currently available technology, marketed grafts and described techniques, these subpar grafts are inserted from a posterior trajectory and do not restore appropriate lumbar lordosis or sagittal balance. Up to now, limited space afforded by a posterior approach because of proximity of critical nerve roots has seemingly and supposedly limited the size and type of graft that has been marketed as available for posterior insertion. Insertion of a smaller and biomechanically misproportioned graft, as necessitated by current grafts that are available for insertion from a posterior approach at this time, may lead to a higher rate of pseudoarthrosis and lower rate of fusion, since surface area contact of endplate vs. graft is less than that obtained during an anterior approach where a larger and more biomechanically appropriate graft can be implanted. Anatomically and biomechanically, the posterior approach to discectomy and fusion, using currently available implants, is further limited in its ability to reestablish optimal lumbar lordosis and restore global sagittal balance (global sagittal imbalance is defined as a distance of greater than 50 mm from the C7 sagittal vertical axis and the S1 sagittal vertical axis), key biomechanical phenomena proven in the literature to be critical to successful patient outcomes. See, e.g., S D Glassman et al., “The impact of positive sagittal balance in adult spinal deformity,” Spine 30(18):2024-9 (2005).
Although the posterior approach to the spine can be faster, easier and less risky in most patients, many surgeons find that the anterior approach to the spine and placement of an Anterior Lumbar Interbody Fusion (“ALIF”) cage is more advantageous because of larger endplate coverage and increased degree of lordosis of the graft that can be implanted, thus achieving greater graft-endplate surface area contact and better biomechanical establishment of lumbar lordosis and restoration of sagittal balance. This procedure is performed from the ventral aspect of the spine, usually through a 3-5 inch incision in the lower left abdominal area. This incision may involve cutting through, and later repairing, the muscles in the lower abdomen, retracting the peritoneum, and retracting the great vessels and ureter. This enables total direct discectomy and placement of a large lordotic cage with generous endplate contact, resulting in excellent fusion rates, and superb reestablishment of lumbar lordosis and sagittal balance. However, the ALIF is somewhat limited in its employment because it requires access to the anterior part of the spine to be provided by a trained vascular or other qualified surgeon before the spine surgeon can perform discectomy and fusion (whereas the posterior approach does not require an approach surgeon). The anterior approach can lead to life-threatening consequences, such as loss of limb and life-threatening hemorrhage, as well as retrograde ejaculation in men, impairing their fertility. The anterior approach may also require further posterior stabilization subsequently, thus leaving patients with an incision from the front of the abdomen and a second incision from the back. Finally, the anterior approach carries a risk of abdominal pseudohernia or true hernia. These potential complications and hassles explain why most spine surgeons prefer the more practical posterior approach and use currently available grafts, but compromise with fusion rates, lordosis reestablishment and sagittal balance correction that is inferior to an anterior approach with a more biomechanically sound graft.
The ideal then would be a graft that can be inserted from a posterior approach (which carries the least risk of morbidity for the patient) but achieves similar benefits to grafts currently available for anterior insertion in terms of restoring sagittal balance and lumbar lordosis. While there are some devices known in the art that are configured for posterior insertion while seeking increased disc and intervertebral height in excess of that traditionally achieved by grafts used in the past during the posterior approach, these devices each suffer from their own limitations and do not correct for the biomechanical shortcomings of current grafts as described above.
There thus remains a need in the art for a device that is configured substantially for posterior insertion (but that can indeed be inserted from an anterior or lateral trajectory as well), and that incorporates the superior biomechanical advantages of an ALIF-type deployment device (e.g., restoring anatomic degree of lumbar lordosis and sagittal balance and improving graft-endplate contact in order to improve fusion rates), while obviating various physiological, mechanical and practical challenges of placement that would otherwise accompany placement from an anterior trajectory. The graft should be placed after thorough and extensive discectomy with minimal risk of harming critical neurovascular structures by way of a protected portal. Described herein is a device and method that simultaneously accomplishes the above.